A Novel Small-Scale Turtle-Inspired Amphibious Spherical Robot IROS2019国际学术会议论文集 1467

Abstract This paper describes a novel small scale turtle inspired Amphibious Spherical Robot ASRobot to accomplish exploration tasks in the restricted environment such as amphibious areas and narrow underwater cave A Legged Multi Vectored Water Jet Composite Propulsion Mechanism LMVWCPM is designed with four legs one of which contains three connecting rod parts one water jet thruster and three joints driven by digital servos Using this mechanism the robot is able to walk like amphibious turtles on various terrains and swim flexibly in submarine environment A simplified kinematic model is established to analyze crawling gaits With simulation of the crawling gait the driving torques of different joints contributed to the choice of servos and the size of links of legs Then we also modeled the robot in water and proposed several underwater locomotion In order to assess the performance of the proposed robot a series of experiments were carried out in the lab pool and on flat ground using the prototype robot Experiments results verified the effectiveness of LMVWCPM and the amphibious control approaches I INTRODUCTION Recently much attention has been focused on autonomous amphibious robots Owing to terrestrial and aquatic ability amphibious robots are widely used in all kinds of high risk tasks such as monitoring and exploration pollution detection search and rescue scientific investigation To achieve multiple locomotion modes in amphibious environment such as snake like wheeled legged oscillatory undulating and propellered researches have proposed various robotic platforms using propulsive mechanisms Different locomotion modes have strength and weaknesses Legged robots cope better with uneven ground but move slowly while wheeled robots 1 walk rapidly but deal poorly with rough terrains snake like robots 2 move well on flat terrains but it is hard to adjust the direction and swimming velocity In water screws propellers based robots are more stable and flexible than those with undulatory and oscillatory capacities Therefore many amphibious robots possess abilities of multiple locomotion modes Kim et al proposed an amphibious robot prototype 3 Using the buoyancy generated by spherical Styrofoam it operates on water and walks on the ground Zhang et al built This work was supported by the National Natural Science Foundation of China 61503028 61773064 Graduate Technological Innovation Project of Beijing Institute of Technology 2018CX10022 and National High Tech Research and Development Program of China No 2015AA043202 Huiming Xing Shuxiang Guo Liwei Shi Xihuan Hou Yu Liu Huikang Liu Yao Hu Debin Xia Zan Li are the Key Laboratory of Convergence Medical Engineering System and Healthcare Technology the Ministry of Industry and Information Technology Beijing Institute of Technology No 5 Zhongguancun South Street Haidian District 100081 Beijing China corresponding authors to provide phones 8686 01068915908 e mails guo eng kagawa u ac jp shiliwei Shuxiang Guo is also with the Faculty of Engineering Kagawa University 2217 20 Hayashi cho Takamatsu Japan e mail guo eng kagawa u ac jp an amphibious robot named AmphiHex I 4 The robot is able to walk on rough terrain and maneuver in water Besides using the novel transformable flipper legs the robot traverses terrain with soft muddy or sandy in amphibious areas Using a novel variable stiffness legs combining the flexible flipper and the rigid fan shaped leg structure Zhong et al developed an improved version AmphiHex II 5 On land and underwater locomotion performance of AmphiHex II was improved highly With an eccentric paddle mechanism based on the epicyclic gear mechanism Shen et al demonstrated an amphibious robot 6 Using the rotational and oscillating paddling methods this robot is able to perform various terrestrial and aquatic gaits Besides these normal amphibious robots biology inspired amphibious robotic has also been researched widely such as snakes fish Natatores 7 and turtles Guo et al proposed a novel geometry mechanics based serpentine gait for snake like robots 8 Crespi et al developed a bio inspired amphibious salamander robot 9 With four legs and an active spine the robot is able to swim in water and walk on ground with various gaits Utilizing a wheel propeller fin mechanism and a specialized swivel mechanism Yu et al designed an amphibious biomimetic fish like robot termed AmphiRobot II 10 The wheel propeller fin mechanism is regarded as a drive wheel for walking on land and as a common screw thruster or pectoral fin in water Vogel et al proposed RoboTerp a quadrupedal amphibious robot 11 with a passive compliant mechanism in the lower leg However the robot cannot sink into the water but walk on land and swim on the water In this brief a novel small scale turtle inspired amphibious spherical robot is proposed with a legged multi vectored water jet composite propulsion mechanism And the robot was able to walk like turtles on different terrains and swim flexibly in water Legged locomotion can be used for the littoral environment With multiple configurations of legs using LMVWCPM ASRobot can realize hovering in water floating and sinking rotational locomotion with zero radius Compared with the previous amphibious robots 12 20 the robot was enhanced as follows 1 The leg was designed with three joints driven by enhanced servomotors The workspace of leg with three joints is larger than that with two joints It is able to improve the flexibility maneuverability and adaptability 2 Improve the sensing abilities by carrying more sensors 3 Using compact design and precision machining improve the ability of waterproof and lose weight of the robot The rest of the paper is organized as follows Section II presents the concept of the robot and introduces LMVWCPM on land model and gaits are given in Section III The force analysis in water and multi locomotion is elaborated in Section IV Experimental results will be addressed in Section V Finally Section VI concludes the paper with an outline of future work A Novel Small scale Turtle inspired Amphibious Spherical Robot Huiming Xing Student Member IEEE Shuxiang Guo Senior Member IEEE Liwei Shi Xihuan Hou Yu Liu Huikang Liu Yao Hu Debin Xia Zan Li 2019 IEEE RSJ International Conference on Intelligent Robots and Systems IROS Macau China November 4 8 2019 978 1 7281 4003 2 19 31 00 2019 IEEE1702 II DESIGN OF THE AMPHIBIOUS SPHERICAL ROBOT A Overall Mechanism In consideration of the benefits of spherical robots such as the stability of mechanical structure the anti disturbance performance the sample kinematics model with three plane symmetry and the strong loading capacity the amphibious spherical robot is built to monitor the littoral environment As depicted in Figure 1 the robot mainly consists of a sealed cabin a top shell that keeps stereo camera Inertial Measurement Unit IMU a multiple pressure sensors based artificial lateral line the communication module two quarter spherical hulls a central plate LMVWCPM and a detachable battery cabin with three batteries Both the sealed cabin and the shell can keep a hemisphere shape And with two quarter spherical hulls the robot keeps a spherical shape A waterproof plug is mounted on the sealed cabin and it connects the robot to remote computer via an optical fiber cable which will assist in debugging the software easily An O ring is utilized between the seal cabin and the central plant to confirm waterproofing In water the robot swims like a ball with the two quarter spherical hulls closed up On land the robot walks with two hulls opened Acoustic communication Top shell Quarter Spherical Shell Stereo camera Pressure sensor Sealed hull Waterproof plug Leg Battery hull Circle PCB Jetson TK1Optical fiber PCB a b Figure 1 The structure of ASRobot a Underwater configuration b on land configuration Assuring that the amphibious robot swims in water with the balance of gravity and buoyancy and walks on land flexibly the most challenge is the rational design of the robot s weight and volume The robot cannot walk with the heavier body smoothly and if the gravity is larger than the buoyancy or less than the buoyancy the robot will sink to the seabed or float to the water surface If the robot requires an extra thrust to maintain hovering in water it will cause an unnecessary energy consumption Considering these aspects the robot is designed using SolidWorks 2017 with powerful functions One function is that the weight of one mechanical part can be calculated by selecting the texture Finally the volume of water discharged from the robot was approximately equal to the actual weight of the robot And the weight of the robot is about 6 6Kg B The Legged Multi Vectored Water jet Composite Propulsion Mechanism The proposed robot relied on LMVWCPM for on land and underwater locomotion As shown in Figure 2 a the composite driving mechanism has the radially free distributed structure This structure is more superiorities than the traditional ones The four cambered slides can keep four mechanical legs swinging smoothly As shown in Figure 2 b the mechanical leg with 3 Degrees of Freedom DoF is composed of three digital servos three connecting rod parts one duct type water jet electric propeller and two bearings which make the leg rotate between the cambered slide and the middle plate steadily and smoothly Three connecting rod parts are termed as coxa femur and tibia respectively And parameters of three connecting rod parts are shown in Table I The joints linking these parts are named as follows the joint linking the body and coxa is Thoraco Coxal joint TC joint and it keeps leg s forward and backward movements the joint linking the coxa and femur is Coxa Trochanteral joint CTr joint which actuates elevation and depression of the leg the joint linking the femur and tibia is Femur Tibia joint FTi joint which drives extension and flexion of the tibia Three joints all are active joints actuated by servos named as Coxa Servo CS Femur Servo FS and Tibia Servo TS respectively Thus this composite driving mechanism enables the robot to crawl more flexibly on land and swim more swiftly in water a b Figure 2 a The LMVWCPM b One mechanical leg III ON LAND MODEL A Forward and Inverse Kinematic Model on Land To describe the locomotion of the robot simply four legs are termed as LF LH RH and RF 17 As shown in Figure 3 a the coordinate system B O lied in the geometrical center of the body The XB YB and ZB axes represent the forward direction the vertical direction perpendicular to the body s horizontal plane and the direction to the right of the body The coordinate system 0 i O represents the mobile base coordinate of the leg and parameter i is leg index 1 for LF 2 for LH 3 for RH and 4 for RF As shown in Figure 3 b the coordinates 1 1 O 1 2 O 1 3 O and 1 4 O are built in TC joint CTr joint FTi joint and the toe of LF 2 1 Y 2 1 O 1 1 Y 1 1 X B X B Y 4 1 X 4 1 Y 3 1 X 3 1 Y 2 1 X 2 1 O B O 1 1 O 4 1 O LFRF LH RH 1 2 X 1 2 O 2d 1 2 Y 1 2 Z 1 3 Y 0a 1 1 O 1 1 Z 1a 1 3 Z 1 3 O 1 3 X 1 4 Z 1 4 O 1 4 Y1 4 X 3d 3a 4d 1 1 Y 1 1 X B X B Y B Z B O a b Figure 3 a The coordinates of ASRobot and legs top view b the coordinates of LF TABLE I THE DENAVIT HARTENBERG D H PARAMETERS OF LF Join t j i j 1 i j 1 i j a i j d i j 1 1 1 0 0 0 a 1 0 d 0 2 2 1 2 0 2 1 97 a 2 10 d 4 3 3 1 3 0 0 0 3 60 d 6 2 4 0 0 3 33 a 4 85 d 0 1703 Table I lists the Denavit Hartenberg D H parameters of LF i j and i j d are the joint angle and the distance between the joint j 1 and j of leg i 1 i j and 1 i j a are the torsional angle and the length of the bar j 1 of leg i respectively Using the D H homogeneous transformation formula in the driven axis context the position of the LF toe in the mobile base coordinates is given by 11 11 11 11 12331 2341 2311 11 11 11 11 12331 2341 2311 1111 233234232 x y z pc c ac s dc s dc a ps c as s ds s ds a ps ac dc dd 1 p where 2 2a sin ii jj s cos ii jj c sin iii jkjk s and cos iii jkjk c i ii jjj cscs 1 2 3 4i and 1 2 3j k Similarly equations of LH RH and RF also is able to be acquired with the same procedure of LF In order to realize the robot movement the inverse kinematic model needs to be built firstly The inverse kinematic equations are derived via the forward kinematic model Given that the toe position of LF is 111 T xyz ppp 1 toe p in O0 the position in other coordinate systems can be obtained by inverse transformation The position in O1 is given by Equation 2 11011 0111 21 31 414234 TTTTT T Then Equation 3 containing two joint variables is obtained by Equation 2 1111 11 1111111 23323423111 1111 233234232 0 xy xy z s pc p c as ds dac ps p s ac dc ddp Define 1111 111xy mc ps pa 22222 3433 2tmnaddd and 1 2z npr 1 1 and 1 3 are obtained by Equation 4 111 1 1222 33434 atan2 atan2 atan2 yx pp a dtadt With this equation 21011111 0121 31 412434 TTTTT T we can get Equation 5 1 111 11111111 1212222213334 1 111 11111111 12122212233343 xyz xyz c c ps c ps ps dc ac as d c s ps s pc ps ac ds ac dd Define 11 3334 c as dk The angle 1 2 is calculated by Equation 6 1222 2 atan2 atan2 m nkmnk Similarly equations of LH RH and RF also can be acquired where IK indicates the inverse kinematics allowing mapping from the Cartesian space to the joint space B Gaits For quadruped robots with a lower body weight many gaits such as walking gaits tripod gaits and trotting gaits are designed mostly However ASRobot is much heavier than other quadruped robots to swim in water It is a huge challenge to support the body with four legs and the leg needs to possess waterproofing Therefore only a crawling gait and a rotary gait were designed which allowed the robot to walk like the turtle The sequence of the crawling gait in one cycle is shown in Figure 4 The gray bar indicates the transfer phase and the black and blue bars show the support phase and moving body phase respectively Over single cycle of the crawling gait the robot moves the body twice in support phase blue bar with four legs Compared to the previous robot ASRobot with three joints has plenty of advantages for flexibility maneuverability and adaptability LF RH RF LH Figure 4 The sequence of the crawling gait and rotary gait Figure 4 also can describe the rotary gait The most difference is the motion direction of the leg If the legs all swing to the left the robot will rotate to the left The robot can rotate to the right with right swing of legs C Simulation with ADAMS To keep the robot crawl steadily and flexibly with its structural design the choice of servomotors and the length of links is essential A virtual simplified model of ASRobot is built and simulated in the ADAMS environment to measure the driving torques of different joints in the sit to stand motion and crawling motion a the driving torque of TC joint b the driving torque of CTr joint c the driving torque of FTi joint Figure 5 The driving torques of different joints in sit to stand motion and crawling with four cycles As shown in Figure 5 the sit to stand motion continues to 1s From these curves the driving torque of TC joint is quite 1704 small and the driving torques of CTr joint and FTi joint are up to 2200 N mm and 1300 N mm The crawling torque curves of different joints in LF is depicted from 1s to 25s Figure 5 and conclusions are drawn as follows First the driving torques of CTr joint and FTi are greater than that of the TC joint this is due to support the heavy body of ASRobot while crawling and the TC joint only swing the mechanical leg The driving torque curves appear abrupt and form spikes which will induce unbalanced moment and impact the stability of ASRobot Second for CTr joint and FTi joint the driving torque in the support phase is much larger than the transfer phase This is because four legs support the body in support phase and three legs support in transfer phase Thirdly during the support phase the maximum driving torque of TC joint basically remains steady and below 750 N mm and the maximum driving torques of CTr joint and FTi joint stay under 2250 N mm and 1000 N mm respectively From simulation results of the sit to stand motion and crawling motion CTr joint requires the largest driving torque about 22 5kg cm The driving torques of TC joint and FTi joint are below 10kg cm In the future other heavy sensors will be installed in the robot servomotors with larger toques in different joints are selected as shown in Table III and the length of leg components is shown in Table II TABLE II LENGTH OF LEG COMPONENTS Name Coxa Femur Tibia Length mm 99 43 62 00 32 75 TABLE III SPECIFICATION OF SERVOMOTORS IN DIFFERENT JOINTS Servomotors CS 6 6V 23 kg cm 0 12 s 60deg FS 8 4V 38 kg cm 0 12 s 60deg TS 6 6V 23 kg cm 0 12 s 60deg Water jet thruster 24V 2A 2 2N Max IV UNDERWATER MODEL As shown in Figure 6 with symmetrical disposition of four legs ASRobot has robust underwater motion under disturbances The force analysis of the vectored propulsion is conducted in the horizontal and vertical propulsion 2 H F 3 H F 4 H F 1 H F 4 Y F 4 1 1 1 1 Y F 2 Y F 2 X F 2 1 3 Y F 3 X F 3 1 4 X F 1 X F B X B Y 1 2 1 3 1 H F 1 F 1 V F a b Figure 6 The force analysis of LMVWCPM a The force analysis of four propellers in horizontal plane b the force analysis of one leg A The Horizontal Propulsion Module In the horizontal plane the leg is driven by CS and it can rotate around its axis within the bound 1 1 4 4 rad The thrust along the X axis 1 1 1222333444 123123123123X FF s cF s cF s cF s c The thrust along the Y axis 1 1 1222333444 123123123123Y FF c cF c cF c cF c c The moment on the Z axis 111222333444 123123123123 Z Tal F cs cFcscFcs cFcsc where 2 2a sin ii jj s cos ii jj c sin iii jkjk s and cos iii jkjk c i ii jjj cscs 1 2 3 4i and 1 2 3j k l is the distance between the CS axis and the center of ASRobot B The Vertical Propulsion Module The vertical forces in Z axis direction of ASRobot is shown in Figure 6 The thrust on the Z axis 1 1223344 23232323Z FF sF sF sF s The moment on the X axi